Microscopic description of acid–base equilibrium

Abstract

Significance Acid–base reactions are among the most important chemical processes. Yet we lack a simple way of describing this class of reactions as a function of the atomic coordinates. In fact, once dissolved in water, H+ and its conjugate anion OH− have a highly fluxional structure difficult to pin down. Here we solve this issue by taking the point of view of describing acid–base reactions as an equilibrium between the solute and the whole solvent. This allows identifying generally applicable descriptors. As a consequence it is now possible to perform quantitative enhanced sampling simulation of acid–base reaction in water and in other environments such as the zeolite cavities or at surfaces. Acid–base reactions are ubiquitous in nature. Understanding their mechanisms is crucial in many fields, from biochemistry to industrial catalysis. Unfortunately, experiments give only limited information without much insight into the molecular behavior. Atomistic simulations could complement experiments and shed precious light on microscopic mechanisms. The large free-energy barriers connected to proton dissociation, however, make the use of enhanced sampling methods mandatory. Here we perform an ab initio molecular dynamics (MD) simulation and enhance sampling with the help of metadynamics. This has been made possible by the introduction of descriptors or collective variables (CVs) that are based on a conceptually different outlook on acid–base equilibria. We test successfully our approach on three different aqueous solutions of acetic acid, ammonia, and bicarbonate. These are representative of acid, basic, and amphoteric behavior.